This invention relates to the manufacture of a chemically basic fused cast refractory of the type having principal crystal phases of periclase and magnesium-spinel, which are often associated with minor silicate phase. Exemplary illustrations of such type of refractory are disclosed in U.S. Pat. No. 3,198,643 and U.K. patent specification No. 965,850.
Solid small lumps, particles, granules, powder, pieces, fragments and the like (generally referred to herein as "grog") have commonly been added to cast molten refractory materials for the purpose of inhibiting the natural formation of relatively large shrinkage cavities during solidification thereof and of inhibiting other problems associated therewith, and thereby providing fused cast products with generally denser central portions. Specific practices implementing this general concept for various different compositions of fused cast refractory are disclosed in: U.S. Pat. Nos. 1,728,350 -- 1,878,870 -- 2,154,153 -- 3,233,994 -- 3,662,058 -- 3,763,302, U.K. patent specification No. 392,364 and "Refractories" by F. H. Norton, 4th ed. 1968 (McGraw-Hill Book Co., New York, page 189. An apparent exception to this purpose is indicated in U.K. patent specification No. 1,344,650, which discloses improving thermal shock resistance of apparently otherwise dense fused cast refractory having periclase and calcium chromite primary crystal phases by the addition of magnesia particles to the cast molten refractory material.
In the particular case of basic fused cast refractory having periclase and magnesium-spinel primary crystal phases, the added grog has customarily been crushed scrap (or cullet) of that same basic fused cast refractory (e.g., see U.S. Pat. No. 3,763,302 and "Refractories" by Norton as cited above). While the resulting products of this latter case exhibit the noted generally denser central portions, many of them have been found to also exhibit one or two other apparently peculiar detrimental characteristics. One of these detrimental characteristics is the limited level of flexural rupture strength (modulus of rupture or MOR) of these fused cast refractories at service temperatures in the range of about 1340.degree. to 1500.degree. C. and particularly at about the latter temperature. The other detrimental characteristic is the formation of shell in these products, especially when cast of molten refractory material produced in a melting furnace operated at or near maximum capacity for volume output of molten refractory material therefrom per unit of time. Shell is a peculiar structural condition in a fused cast refractory product that develops during solidification thereof. It is the result of the formation of a substantial or at least semi-continuous band, layer or stratum of void spaces or cavities located a relatively short distance beneath and extending substantially parallel to the cast surfaces of the fused cast product which are formed adjacent to mold cavity surfaces. Such layer of cavities delineates overlying, relatively thin cast surface or "skin" portions substantially apart from the main body of the fused cast product. Those portions, referred to as "shell", are or become easily broken off the cast product upon its removal from the mold and/or during necessary subsequent handling of it. Those products with shell are generally not commercially saleable nor otherwise useful except as scrap for reuse as grog in the fusion casting process or as grain in rebonded fused grain bricks.